The present invention relates to a variable inductor. More particularly, the present invention relates to an apparatus and method for orthogonal inductance variation.
Inductors possess the ability to store energy in their electromagnetic fields. This property has made inductors an important component in several categories of electrical circuits. As an example, inductors are important components in power conversion equipment, oscillators, and filters. In power conversion equipment, inductors are used in circuits which provide voltage rectification. Also, inductors are used in a variety of electrical devices such as voltage controlled oscillators, amplifiers, modulators, tuning circuits, and filters. In these and other embodiments, the natural resonant frequency of an oscillator or the cut-off frequency of a filter is determined, in part, by the combination of capacitors and inductors used in those circuits. In some instances, inductor inductance can be intentionally varied such as by mechanically changing the physical size of the core air gap. However, these mechanical methods have drawbacks such as the need for additional parts, complexity and bulk.
The inductors in these tunable devices have long been considered static inductance inductors, and this mindset has stifled growth and improvement in many electronics devices. This is particularly true of low voltage and high current power conversion devices. In one particular example, the demand for higher performance, microcontroller-based products for use in communication and processing applications continues to increase rapidly. As a result, microcontroller-based product manufacturers are requiring the components and devices within these products to be continually improved to meet the design requirements of a myriad of emerging audio, video and imaging applications. Microcontroller""s are being designed with increasingly higher load demands and with lower voltage requirements. For example, many microprocessors are now designed to operate with a 3V power supply, and others are designed to work with less than a 1V power supply. This trend towards designing integrated circuits to operate at lower voltage levels is likely to continue. However, efficient power converters are increasingly difficult to design at these lower voltage levels.
Generally, AC power is converted to a steady DC power supply for microcontroller use. Furthermore, DC power is transformed from one voltage level to another through power converters. High efficiency power conversion is increasingly difficult to achieve as power converter output voltage requirements decrease and load current demands increase. This difficulty is largely due to the dominant conductive and switching losses of the output rectifiers. In prior efforts to improve the efficiency of the power conversion, standard rectifier diodes were replaced with synchronous field effect transistor (xe2x80x9cFETxe2x80x9d) rectifiers. These FET based systems, also known as synchronous forward converter""s (xe2x80x9cSFCxe2x80x9d), are inefficient at low voltages with high current, and when output voltages on the order of 1 Volt or less are desired, a better rectification method is needed.
An exemplary integrated circuit device using a non-variable inductor may, for example, include a synchronous FET rectifier. Synchronous FET rectifiers are used, for example, in a synchronous forward converter system 100, as shown in FIG. 1. SFC system 100 has a power source 102 and a load 116. SFC system 100 also has a transformer 104 with a secondary winding 122, a reset winding 123 and a transformer reset diode 106. SFC system 100 also includes a primary switch 108, an output rectifier switch 110, a freewheeling rectifier switch 111, an output inductor 112, output capacitance 114, and a feedback control circuit 118. In typical operation, source 102 is a DC power source providing DC source voltage to the transformer 104. Alternating ON and OFF states provided by controller 118 and primary switch 108 result in the generation of AC voltage. FET switches 108, 110, and 111 are synchronized by controller 118.
During an xe2x80x9cONxe2x80x9d state, primary switch 108 and output rectifier switch 110 are both configured to be on while the freewheeling switch 111 is configured to be off. During the ON state, voltage on secondary winding 122 of transformer 104 produces a positive voltage proportional to the primary side voltage. This voltage is a function of the turns ratio of transformer 104. During the ON state the secondary winding 122 voltage minus the steady state load 116 voltage is applied across the inductor 112. This results in a linear increase of current in inductor 112.
During an xe2x80x9cOFFxe2x80x9d state, primary switch 108 and output rectifier switch 110 are configured to be off while the freewheeling switch 111 is configured to be on. Under this condition, magnetic forces within transformer 104 force the voltages on all windings to reverse polarity. These magnetic forces in conjunction with reset diode 106 facilitate reset of the transformer core to prevent saturation of the core material and subsequent loss of efficient transformer action. Because rectifier switch 110 is in the OFF state, the secondary winding 122 voltage is allowed to produce a negative potential in order to facilitate transformer 104 reset, without impacting power delivery to the load. Because freewheeling switch 111 is in the ON state, node 120 is coupled to the ground potential. This results in maintenance of current flow direction in output inductor 112. During the OFF state the equivalent voltage across the inductor 112 is 0 minus the load 116 voltage resulting in a linear decrease of current in output inductor 112.
The voltage and current ripple produced by the linear ramping of current in output inductor 112 is filtered by output capacitor 114 to produce DC current to load 116. In this manner, output rectifier switches 110 and 111 are synchronized with the operation of primary switch 108; however, this synchronization is a significantly complicated task. Accordingly, a need exists for a less complex method of operating a forward converter.
The average voltage value supplied to the load may also be regulated by SFC system 100 by varying the duty cycle with feedback control device 118. For example, device 118 can vary the percentage of time that the positive voltage is provided to the input node 120 of output inductor 112, in other words, changing the amount of time the power to the load is xe2x80x9coffxe2x80x9d. Reducing the duty cycle, reduces the DC voltage at the load and thus regulates the output voltage. The steady state transfer relationship for the forward topology is:                               V          ⁢                      xe2x80x83                    ⁢          o          ⁢                      xe2x80x83                    ⁢          u          ⁢                      xe2x80x83                    ⁢          t                =                  V          ⁢                      xe2x80x83                    ⁢          i          ⁢                      xe2x80x83                    ⁢          n          ⁢                      xe2x80x83                    ⁢          D          ⁢                                    N              ⁢                              xe2x80x83                            ⁢              s                                      N              ⁢                              xe2x80x83                            ⁢              p                                                          (        1        )            
Where:
Np=Transformer Primary # of Turns
Ns=Transformer Secondary # of Turns
SFC system 100 is inefficient at low voltages with high current. Furthermore, increasing the number of rectifiers to parallel the equivalent resistance results in diminishing returns due to       1    2    ⁢      CV    2  
and gate drive current losses. These energy losses are expensive, give rise to increased heat generation/removal issues, and impact the reliability of the device due to increased possibility of burn out of the rectifier. When SFC system 100 is operated at low voltage and high current, the bulk of the loss is concentrated in conducted and switching loss within the output rectifiers 110 and 111. Due to the placement of output rectifiers 110 and 111, current flows through one of the two devices at all times, and all current that reaches load 116 flows through these devices. The losses can be significant, and a need exists for an efficient rectifier which can regulate output voltage and can do so without the high power losses of the prior art.
Demand also exists for efficient and/or smaller power converters which can operate under low voltage/high current conditions in exemplary devices such as some high power laser diodes used in the telecommunication industry and arc welders. The use of non-variable inductors has also stifled development in other electronics areas, for example, inductors are used in combination with resistors and/or capacitors in circuits to form oscillators and filters. Non variable inductors are used in a variety of electrical devices such as power converters, rectifiers, voltage controlled oscillators, amplifiers, modulators, tuning circuits, filters, etc. In these designs, the natural resonant frequency of an oscillator or the cut-off frequency of a filter is set by providing set inductance and set capacitance values. However, often it is desirable to vary the resonant frequency or the cut-off frequency. To accomplish this variation, the circuits are configured to vary the capacitance of the capacitors. These variable capacitors may include trim capacitors and varactor junction diodes. Furthermore, banks of capacitors may be used to make large changes in overall capacitance by combining capacitors in parallel and in series. Each of these methods of varying the capacitance is expensive, requires extra circuitry and parts and is subject to additional failures. Furthermore, as semiconductor components, the capacitors are lossey elements with poor efficiencies. Therefore, there exists a need for more efficient methods of tuning the resonant frequency and cut-off frequency, and for a less complicated way of and ability to perform fine tuning.
The method and device according to the present invention addresses many of the shortcomings of the prior art. In accordance with one aspect of the present invention, a control system, method and apparatus are provided for varying the inductance of an inductor using orthogonal magnetic interference. In an exemplary embodiment, the orthogonal magnetic interference is generated by, for example, an external inductance (xe2x80x9cHxe2x80x9d) field device, a series method orthogonal flux device, or a combined core device.
In accordance with another aspect of the present invention, a control system, method and apparatus is provided for altering an AC voltage for a DC load using a variable inductor. In an exemplary embodiment of the present invention, an orthogonal inductive device is provided to facilitate varying the inductance in the output current path. In a further exemplary embodiment, the orthogonal inductive device is, for example, an external H field device, a series method orthogonal flux device, or a combined core device. In accordance with another aspect of the present invention, DC voltage regulation is also provided by use of a variable inductor. In a further aspect of the present invention, regulation is provided without the use of silicon devices, such as FET""s, in the output current path. In accordance with other aspects of the present invention, efficient voltage regulation is provided by varying the inductance of a device in the output current path, and alternatively by varying both the inductance and duty cycle.
In accordance with further aspects of the present invention, a filter apparatus and method is provided for variably tuning the cut-off frequency of the filter using a variable inductor. In accordance with another aspect of the present invention, an oscillator apparatus and method is provided for variably tuning the natural resonant frequency of the oscillator using a variable inductor.